Never had this happen but have heard of it happening to others:
Replace the flexible lines to the wheels.
Those flex lines can have a portion come loose internally that blocks
the flow of hydraulic fluid, so after applying the brakes for a while
they won't release because the pressure can't escape back through those
lines.
A good mechanic should be familiar with this phenomenon.

If your mechanic has never heard of dragging brakes and can't diagnose you
need a new mechanic. If he's never heard of a vacuum booster causing a brake
drag then he may not be so bad as a lot of mechanics haven't.
As another poster suggested, start by bleeding your brakes. Bleed in the
order RR, LR, RF and then LF. Bleed until fluid runs clear, monitoring fluid
level in master cylinder to make sure you don't get air in lines.
This is a good time to check slides or slide pins on calipers to see if they
are dry. If so, this can cause drag, brake lube will solve this.
When you test drive, if the brakes drag again, try pulling up on the brake
pedal with your foot. If this releases the drag then you probably have a bad
master cylinder or booster. More likely a master cylinder. Usually when the
booster is bad you can hear a vacuum leak and if you disconnect the vacuum
then the problem goes away.
If not then when brakes start to drag, stop and crack each bleeder screw. If
there is a lot of pressure and brake fluid sprays out under pressure then
the flexible hose is bad internally causing pressure to hold.
HTH, davidj92